PTOs have long been used on many types of vehicles and work machines, including on agricultural vehicles such as combines and tractors, to provide power for equipment or implements, such as, for agricultural purposes, mowers, balers, forage harvesters and spreaders. Hereinafter, for the present purposes the terms vehicle and work machine are considered the same and are used interchangeably. PTOs are typically selectably connectable to a source of rotational power such as an engine of the vehicle by a clutch, such as an electrical or fluid controlled clutch, controlled by a clutch control system. Many known PTO clutch control systems have been developed over the years, and have operated under a variety of control strategies designed to provide desired features such as smooth engagement and to protect PTO shafts from catastrophic failure and attached equipment during the clutch engagement from damage.
Typical of such known systems is the system of U.S. Pat. No. 5,494,142, which discloses a PTO control system for vehicles, such as farm tractors including a PTO shaft, for supplying rotational motion to an implement of the type which may be stationary or towed by the tractor. Power is transferred to the PTO shaft by a clutch including an input shaft coupled to a power source and an output shaft coupled to the PTO shaft. The clutch transmits a maximum torque between the input and output shafts in response to a maximum clutch pressure and transmits a variable torque between the input and output shafts in response to a given clutch engagement pressure that is less than the maximum clutch engagement pressure.
Such control system includes a clutch control for engaging and disengaging the clutch in response to first and second control signals, respectively, and transmits a variable torque between the input and output shafts dependent upon a given clutch engagement pressure defined by the first control signals. The clutch engagement pressure is less than the maximum engagement pressure to limit the torque transfer between the input and output shafts by the clutch. The control system also includes a first transducer disposed to generate an input signal representative of the rotational speed of the input shaft, a second transducer disposed to generate an output signal representative of the rotational speed of the output shaft, and a control circuit. The control circuit is coupled to the clutch control, the first transducer, and the second transducer.
While such a control system has been of great value and effectiveness, it and other known control systems have continued to experience difficulties when attempts are made to drive loads with the PTO which are extremely light or heavy. With such systems, one shortcoming is that no differentiation is made with respect to the loads applied, be they very light or very heavy. With a light applied load, initial PTO shaft movement could occur at a relatively early time and modulation could be achieved over a relatively longer time period than necessary. With a heavy load, however, initial PTO shaft movement would not occur until a later time, leaving much less time for modulation.
Also reference U.S. Pat. No. 6,267,189, which discloses another known method for controlling engagement of a PTO clutch. In this method, during engagement of the PTO, a control circuit continuously determines an actual acceleration of the PTO based upon the output shaft velocity, and generates control signals using a desired acceleration and the actual acceleration. The control circuit may continuously determine the desired acceleration based upon the input speed signal. Alternatively, the control circuit may provide control signals to cause the torque transmitted through the clutch to increase at a rate depending upon the difference between the desired and actual accelerations when the actual acceleration is more than a threshold proportion of the desired acceleration, and to cause the torque to increase at a rate independent of the difference when the actual acceleration is less than the threshold proportion.
Additionally, the system of U.S. Pat. No. 6,267,189 may make adjustments that can adversely effect smooth clutch engagement. For instance, under some heavy loading conditions, the resulting applied torque can result in substantial engine droop, but no, or very little, rotation or acceleration of the PTO output shaft. Under such conditions, a problem that can occur is that the system may categorize the actual acceleration as below the threshold value, and proceed to cause the torque to increase at a faster rate, possibly resulting in problems such as rough engagement, engine stalling, and/or increased component wear and/or damage, due to the high torque conditions.
Thus, what is sought is a PTO control system which overcomes one or more of the problems and shortcomings set forth above.